Faraday optical current sensors may be used for measuring a current in a nearby power line. Faraday optical current sensors rely on the Faraday effect. The Faraday effect states that the rotation of a polarized light beam is proportional to the magnetic field component in the direction of the beam. A charge moving inside a conductor will produce a circular magnetic field around the conductor. Thus, by placing an optical Faraday optical current sensor parallel to the direction of the magnetic field lines the magnitude of the current may be measured.
Using a Faraday optical current sensor provides many advantages compared to conventional technologies such as current transformers. One of the most important advantages is the fact that the Faraday optical current sensor may be constructed entirely from dielectric materials. This is especially important for high voltage/high current applications and gives the Faraday optical current sensor substantial immunity against electromagnetic disturbances. Another important advantage of Faraday optical current sensors is that they may be galvanic separated from the power line and they do not influence the current in the power line in any way. This almost eliminates the risk of a short circuit of the power line thought the measurement system. One example of such a Faraday optical current sensor is the DISCOS® Opti module produced by the applicant company and described in U.S. Pat. No. 7,068,025, to which reference is made and which is hereby incorporated in the present specification by reference.
A Faraday optical current sensor comprises a magneto-optical part typically formed as a rod, fibre or similar made of a material exhibiting a high Faraday effect. This typically means a material having a high Verdet constant. The Verdet constant is the proportionality constant of the Faraday effect. The angle of rotation of the polarized light may be described by the following formula:β=V×B×d where β is the angle of rotation, d is the length of the path where magnetic field and light interact, B is the magnetic flux density in the direction of light propagation and V is the Verdet constant. The magnetic flux density at a certain location outside a conductor may be calculated by using the well-known formula:
  B  =                    μ        0            ⁢      I              2      ⁢                          ⁢      π      ⁢                          ⁢      r      where B is the magnetic flux density, μ0 is the magnetic constant, I is the current and r is the distance from the conductor.
The magneto-optical part may be supplied with polarized light from a light source such as a lamp or LED emitting linear polarized light in a specific wavelength. The light source may comprise a polarized filter for generating light with a specific linear polarisation. The light exiting the magneto-optical part may be detected and preferably converted to an electrical signal by a detection unit. The detection unit detects the rotation of the polarized light exiting the magneto-optical part. A control unit may evaluate the signal from the detection unit, perform the necessary error corrections and calculations to determine the current in the power line. Possible sources of errors include sensor position in relation to the power line, optical noise, transition effects when light enters and exits different optical media and temperature effects. The Faraday optical current sensor is preferably calibrated before use, e.g. by using standard current measurement equipment. Standard current measurement equipment may comprise e.g. a current transformer. After calibration the Faraday optical current sensor may replace e.g. a current transformer for monitoring currents and report the measured values to a control system. The Faraday optical current sensor may also be used to detect fault currents such as short circuit currents and report such occurrences to a safety system, which may in turn activate the relevant circuit breakers and backup systems to avoid damage to other equipment in the power distribution grid.
The magneto optical part and the light source and the detection unit are preferably connected via an optical conduit such as an optical fibre. Optical fibres provide a substantial amount of flexibility and allow light to travel long distances without considerable losses in light intensity. However, it is important to be aware of the limits in flexibility of optical fibres. Optical fibres may fail due to being broken, damaged or deformed if they are bent beyond a flexibility limit. A failure in the optical fibre due to excessive bending will typically permanently make them unusable for conducting light. Typical optical fibres may be bent considerably less than electrical cables.
Since optical sensors may be constructed by using dielectric materials only the sensors may be positioned in locations where other sensors, i.e. sensors comprising conductive materials, are not suitable. Such locations include places subject to high electrical fields, which are common in the field of high current and high voltage engineering. Additionally, the Faraday optical current sensors are very compact and light since they do not contain any metal parts. The magneto-optical part for high voltage and high current applications may be made having dimensions in the mm range. For better handling and protection, the magneto-optical part as well as the junctions with the optical conduits are encapsulated by a small cylindrical housing. All of the above features of the optical Faraday optical current sensors make a broad range of new measurement positions feasible.
New measurement positions require suitable fixation equipment for fixating the Faraday optical current sensor to the power line. In many cases the current measurement equipment constitutes a separate unit in e.g. a substation. Having a separate current measurement unit requires a considerable amount of space and material. However, using the Faraday optical current sensor a separate current measurement unit is not necessary. Due to the small size and dielectric properties of the Faraday optical current sensor it may also be combined with any other high voltage or high current equipment.
Due to the large influence of the measurement position on the measurement results it is important that the measurement position is clearly defined and determined. Also, the measurement position should be protected from tampering and involuntary disturbances.
For outdoor applications such as overhead lines the Faraday optical current sensor should be firmly fixated in the measurement position and at the same time protected against any influence from the nature. For indoor applications such as inside a substation the Faraday optical current sensor should be at least firmly fastened in the measurement position.